[css-shapes-1] Mark up <shape-box> production

Author: heycam

css-shapes-1/Overview.bs

@@ -749,7 +749,7 @@ Shapes from Box Values</h2>
 	to include ''margin-box''.
 	Its syntax is:
 
-	<pre>
+	<pre class="prod">
 		<dfn><<shape-box>></dfn> = <<box>> | ''margin-box''
 	</pre>